In general, in a CT examination of a heart, motion artifacts appear in a tomographic image by the effect of a heartbeat, which results in an unpreferable image in diagnosis. To reduce the motion artifacts, there has been proposed to reduce the effect of the heartbeat by increasing a scan speed as compared with one heartbeat cycle. However, a beat rate of a heart of a human body is different depending on persons and on a physical condition and a mental condition of a person at the time even if they are the conditions of the same person.
Accordingly, heretofore, there is proposed a method of executing imaging by adding a heartbeat waveform to a scan data in asynchronism with a heartbeat cycle and rearranging an image by combining projection data having the same heartbeat time phase (hereinafter, referred to as (cardiac time phase) in Japanese Unexamined Patent Application Publication No. JP2001-137232A. In this publication, a heartbeat cycle of a subject is measured, projection data is acquired by scanning the heart of a subject at an angular speed that is in asynchronism with the measured heartbeat cycle, and a tomographic image of the heart of the subject is generated from of projection image segments that are discrete in time. The generated image corresponds to a selected portion of the heartbeat cycle, for example, to a relatively static portion thereof.
That is, the publication discloses to reduce motion artifacts due to the effect of a heartbeat by acquiring a tomographic image of a heart by rearranging the image of projection data imaged in a cardiac time phase, in which the motion of the heart is minimized (hereinafter, referred to as static cardiac time phase), using heartbeat information. Here, the cardiac time phase is a concept of a heartbeat cycle that is divided into a plurality of sections. The cardiac time phase can be expressed by equally dividing an interval of an R wave, which appears periodically in a heartbeat waveform, into “n” time sections (n is an integer) and referring the equally divided time sections as cardiac time phases so that the respective cardiac time phases can be sequentially expressed from the R wave acting as a reference. Otherwise, the cardiac time phase can be also expressed by a percentage by setting an R wave cycle to 100% and expressing the cardiac time phase by a position on a time axis from the R waveform acting as the reference. That is, when the static cardiac time phase is expressed, it can, be expressed by a relative position (%) of it from the R wave acting as the reference with the R wave cycle expressed by 100% as shown in FIG. 2.
However, a cardiac time phase that is less affected by a heartbeat is different depending on a portion of a heart or on a portion in the vicinity of the heart. Accordingly, there is a possibility that motion artifacts are generated unless an appropriate static cardiac time phase is determined according to an observing portion and unless an image is rearranged using projection data corresponding to the static cardiac time phase. Further, as described above, the static cardiac time phase varies depending on a patient to be imaged and on a health condition of the patient. Accordingly, a tomographic image of a heart with a small amount of motion artifacts may not be acquired in a static cardiac time phase determined by an apparatus. In this case, it is contemplated to generate a plurality of tomographic images in a different cardiac time phase at the same measuring position and to select a tomographic image with a small amount of motion artifacts from the plurality of tomographic images. However, it takes a long time to generate the plurality of tomographic images, and it is time-consuming for an operator to select an image with a small amount of motion artifacts from the plurality of tomographic images.